WO2010098638A2 - Lithium secondary battery comprising a negative electrode with a water-based binder - Google Patents

Abstract

The present invention relates to a lithium secondary battery comprising a negative electrode, a positive electrode, and a non-aqueous electrolyte. The negative electrode contains a water-based binder, and the non-aqueous electrolyte contains: (a) a cyclic anhydride or a derivative thereof; and (b) an anion receptor selected from the group consisting of borane compounds, borate compounds, and a mixture thereof. According to the present invention, the negative electrode is provided with a stable SEI film, and the amount of LiF in the SEI film is controlled, thereby lengthening the lifespan of the battery.

Description

Lithium secondary battery having a negative electrode including an aqueous binder

This application claims the priority based on Korean Patent Application No. 10-2009-0016358 filed February 26, 2009 and Korean Patent Application No. 10-2010-0017594 filed February 26, 2010, All content disclosed in the specification and drawings of an application is incorporated in this application.

The present invention relates to a lithium secondary battery having a negative electrode using an aqueous binder.

Recently, interest in energy storage technology is increasing. As the field of application extends to the energy of mobile phones, camcorders, notebook PCs, and even electric vehicles, the demand for high energy density of batteries used as power sources for such electronic devices is increasing. Lithium secondary batteries are the batteries that can best meet these demands, and research on these is being actively conducted.

A lithium secondary battery has a negative electrode, a positive electrode, and a nonaqueous electrolyte that provides a migration path between lithium ions therebetween, and oxidation and reduction reactions when lithium ions are intercalated / deintercalated at the positive electrode and the negative electrode By generating electrical energy.

As a negative electrode of the lithium secondary battery, a binder is used to bind the active material particles to maintain a molded body. A solvent-based binder (i.e., a binder using an organic solvent as a solvent) represented by polyvinylidene fluoride (PVdF) and styrene- Butadiene rubber (styrene-butadiene rubber, hereinafter referred to as SBR) is divided into an aqueous binder (ie, water-based binder). Aqueous binders, unlike solvent binders, are economical, environmentally friendly, and harmless to the health of workers. In addition, since the binding effect is also greater than that of the solvent-based binder, the ratio of the active material per volume can be increased, and high capacity can be obtained.

However, such a lithium secondary battery has a problem in that performance deteriorates as charging and discharging are repeated. This problem becomes more serious as the capacity density of the battery increases.

In order to solve the above-mentioned problems, methods for forming a solid electrolyte interface (SEI) film on the negative electrode surface by adding various compounds to the nonaqueous electrolyte have been proposed. However, researches on nonaqueous electrolytes optimized for lithium secondary batteries having a negative electrode using an aqueous binder have continued, such as forming a stable SEI film and remarkably improving life characteristics.

Therefore, the problem to be solved by the present invention is to be applied to a lithium secondary battery having a negative electrode using an aqueous binder to form a stable SEI film on the negative electrode, by controlling the amount of LiF in the formed SEI film can improve the life characteristics of the battery The present invention provides a lithium secondary battery having a nonaqueous electrolyte.

In order to solve the above problems, according to the present invention, a lithium secondary battery comprising a negative electrode, a positive electrode and a nonaqueous electrolyte, the negative electrode comprises an aqueous binder, the non-aqueous electrolyte, (a) a cyclic anhydride or derivatives thereof; And (b) any one anion receptor selected from the group consisting of borane compounds, borate compounds, and mixtures thereof.

In the lithium secondary battery of the present invention, the aqueous binder is preferably SBR (styrene-butadiene rubber).

In the lithium secondary battery of the present invention, the cyclic anhydride or derivative thereof is preferably any one selected from the group consisting of compounds represented by the following formulas (1) to (4) or a mixture of two or more thereof.

Formula 1

Formula 2

Formula 3

Formula 4

In Formulas 1 to 4, R ₁ to R ₁₁ are each independently halogen or an alkyl, alkenyl or alkoxy group having 1 to 10 carbon atoms unsubstituted or substituted with halogen.

In addition, in the lithium secondary battery of the present invention, it is preferable that the borane compound uses a compound represented by the following formula (5), and the borate compound uses a compound represented by the following formula (6).

Formula 5

In Formula 5, R ₁₂ to R ₁₄ are each independently hydrogen or halogen, or an alkyl group or silyl group having 1 to 6 carbon atoms.

Formula 6

In Formula 6, R ₁₅ to R ₁₇ are each independently hydrogen or halogen, or an alkyl or silyl group having 1 to 6 carbon atoms.

In the lithium secondary battery of the present invention, the content of the (a) component and (b) component may be 0.05 to 10% by weight, respectively, based on the total weight of the nonaqueous electrolyte.

The lithium secondary battery according to the present invention has the following effects.

First, it is economically and environmentally friendly by using an aqueous binder as a negative electrode binder. In addition, since the binding effect is greater than that of the solvent-based binder, it is possible to increase the ratio of the negative electrode active material per volume, thereby increasing the capacity.

Second, the cyclic anhydride or derivative thereof contained in the nonaqueous electrolyte forms a stable SEI film on the surface of the negative electrode. In addition, the anion receptor of the borane compound or the borate compound elutes the LiF in the SEI film which increases with the use of the aqueous binder and the cyclic anhydride (or derivative thereof) to control the LiF content in the SEI film. Accordingly, the resistance of the SEI film is controlled to improve the life characteristics of the battery.

1 is an XPS graph obtained from a surface of a negative electrode after charging a battery according to Example 1 and Comparative Example 1 at 0.1C, respectively.

Hereinafter, the present invention will be described in detail. The terms or words used in this specification and claims are not to be construed as limiting in their usual or dictionary meanings, and the inventors may appropriately define the concept of terms in order to best explain their invention in the best way possible. It should be interpreted as meaning and concept corresponding to the technical idea of the present invention based on the principle that the present invention.

According to the present invention, a lithium secondary battery comprising a negative electrode, a positive electrode, and a nonaqueous electrolyte, the negative electrode comprises an aqueous binder, the nonaqueous electrolyte, (a) a cyclic anhydride or derivatives thereof; And (b) any one anion receptor selected from the group consisting of borane compounds, borate compounds, and mixtures thereof.

As described above, the lithium secondary battery of the present invention is manufactured economically and environmentally friendly by using an aqueous binder as the binder of the negative electrode. In addition, since the binding effect is greater than that of the solvent-based binder, it is possible to increase the ratio of the negative electrode active material per volume, thereby increasing the capacity. The aqueous binder is preferably SBR (styrene-butadiene rubber), and, as is well known, may be applied to a negative electrode by dispersing it in water together with a thickener such as carboxymethyl cellulose (CMC).

Moreover, in the lithium secondary battery of this invention, a nonaqueous electrolyte solution contains (a) cyclic anhydride or its derivative (s). The cyclic anhydride or its derivatives ring-open at a potential lower than the solvent at the time of initial charge to form an SEI film on the surface of the cathode.

Such cyclic anhydrides or derivatives thereof are preferably any one selected from the group consisting of compounds represented by the following formulas (1) to (4) or mixtures of two or more thereof.

<Formula 1>

<Formula 2>

<Formula 3>

<Formula 4>

In Formulas 1 to 4, R ₁ to R ₁₁ are each independently halogen or an alkyl, alkenyl or alkoxy group having 1 to 10 carbon atoms unsubstituted or substituted with halogen.

Although the SEI film formed from the above-mentioned component (a) has high stability, it contains a large amount of LiF and acts as a large resistance during charge and discharge. The reason why the LiF content in the SEI film is greatly increased is that in addition to the cause of using the component (a), an aqueous binder is used as the negative electrode binder. In other words, unlike a solvent-based binder such as PVdF, the negative electrode using the aqueous binder has a high water content. As a result, the hydrofluoric acid content in the nonaqueous electrolytic solution is significantly increased, so that the amount of LiF is also increased when the SEI film is formed by the component (a).

   The lithium secondary battery of the present invention solves this problem by adding any one anion receptor selected from the group consisting of (b) a borane compound, a borate compound, and a mixture thereof to the nonaqueous electrolyte. That is, an anion receptor made of a borane compound or a borate compound contained in the nonaqueous electrolyte elutes LiF in the SEI film increased with the use of an aqueous binder and a cyclic anhydride (or a derivative thereof). Accordingly, since the LiF content in the SEI film is controlled to be low, the resistance of the SEI film is lowered, thereby improving the life characteristics of the battery.

In the lithium secondary battery of the present invention, it is preferable to use a compound represented by the following formula (5) as the borane compound added to the nonaqueous electrolyte and a compound represented by the following formula (6) as the borate compound.

<Formula 5>

In Formula 5, R ₁₂ to R ₁₄ are each independently hydrogen or halogen, or an alkyl group or silyl group having 1 to 6 carbon atoms.

<Formula 6>

In Formula 6, R ₁₅ to R ₁₇ are each independently hydrogen or halogen, or an alkyl or silyl group having 1 to 6 carbon atoms.

The above-mentioned components (a) and (b) are preferably added in an amount of 0.05 to 10% by weight, for example, based on the total weight of the nonaqueous electrolyte, in consideration of the life improvement effect and performance of the battery.

In the lithium secondary battery of the present invention, the nonaqueous electrolyte contains an organic solvent. The organic solvent is not particularly limited as long as it is usually used as an organic solvent for nonaqueous electrolyte, and cyclic carbonate, linear carbonate, lactone, ether, ester, acetonitrile, lactam, and / or ketone can be used.

Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), fluoroethylene carbonate (FEC), and the like. Examples of the linear carbonate include diethyl carbonate (DEC) and dimethyl carbonate. (DMC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), and the like, and these may be used alone or in combination of two or more thereof. Examples of the lactone include gamma-butyrolactone (GBL), and examples of the ether include dibutyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane And 1,2-diethoxyethane. Examples of such esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, butyl propionate, methyl pivalate and the like. In addition, the lactam includes N-methyl-2-pyrrolidone (NMP) and the like, and the ketone includes polymethylvinyl ketone. In addition, a halogen derivative of the organic solvent may be used, but is not limited thereto. These organic solvents can be used individually or in mixture of 2 or more types.

In the lithium secondary battery of the present invention, the nonaqueous electrolyte includes an electrolyte salt, and the electrolyte salt is not particularly limited as long as it is usually used as an electrolyte salt for nonaqueous electrolyte.

The electrolyte salt is ^{(i) Li +, Na +} , a cation and (ii) selected from the group consisting of _{^{K + PF 6 -, BF 4}} -, Cl -, Br -, I -, ClO 4 -, AsF 6 -, _{^{CH 3 CO 2 -, CF 3}} SO 3 -, N (CF 3 SO 2) 2 -, C (CF 2 SO 2) 3 - , but can be configured with a combination of an anion selected from the group consisting of, but not always limited thereto. These electrolyte salts can be used individually or in mixture of 2 or more types. In particular, the electrolyte salt is LiPF _6, LiBF _4, LiSbF _6, LiAsF _6, LiClO ₄ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , CF ₃ SO ₃ Li and LiC ( It is preferable to use lithium salts such as CF ₃ SO ₂ ) _{3 and} LiC ₄ BO ₈ .

Meanwhile, the lithium secondary battery of the present invention includes all conventional lithium secondary batteries, such as a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery or a lithium ion polymer secondary battery.

The lithium secondary battery of the present invention can be prepared according to conventional methods known in the art. For example, a porous separator may be placed between the positive electrode and the negative electrode to prepare a nonaqueous electrolyte.

The electrode of a lithium secondary battery can be manufactured by conventional methods known in the art. For example, a slurry may be prepared by mixing and stirring a solvent, a binder, a conductive material, and a dispersant in an electrode active material, and then applying (coating) to a current collector of a metal material, compressing, and drying the electrode to prepare an electrode. have.

The positive electrode active material may be _a lithium transition metal composite oxide such as LiM _x O _y (M = Co, Ni, Mn, Co _a Ni _b Mn _c ) (for example, lithium manganese composite oxide such as LiMn ₂ O ₄ , LiNiO _2, etc.). Lithium cobalt oxides such as lithium nickel oxide, LiCoO ₂ , and manganese, nickel, and cobalt in which some of these oxides are substituted with other transition metals, or vanadium oxide containing lithium, etc.), but are not limited thereto. .

The negative electrode active material may be a conventional negative electrode active material that can be used in the negative electrode of a conventional lithium secondary battery, non-limiting examples of lithium metal, lithium alloy, carbon, petroleum coke that can occlude and release lithium ions ), Activated carbon, graphite, carbon fiber, and the like. In addition, lithium oxide may be occluded and released, and metal oxides such as TiO ₂ , SnO _2, and the like having a potential of less than ₂ V may be used, but are not limited thereto. In particular, carbon materials such as graphite, carbon fiber and activated carbon are preferable.

The current collector of the metal material is a metal having high conductivity, and any metal can be used as long as the slurry of the electrode active material can be easily adhered and is not reactive in the voltage range of the battery. Non-limiting examples of the positive electrode current collector is a foil produced by aluminum, nickel or a combination thereof, and non-limiting examples of the negative electrode current collector is produced by copper, gold, nickel or copper alloy or a combination thereof Foil and the like.

Examples of the positive electrode binder that can be used include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and the like, and styrene butadiene rubber (SBR) is used as the negative electrode binder.

The conductive material is not particularly limited as long as it is an electronic conductive material that does not cause chemical change in the electrochemical device. In general, carbon black, graphite, carbon fiber, carbon nanotubes, metal powder, conductive metal oxide, organic conductive materials, and the like can be used, and currently commercially available products as acetylene black series (Chevron Chemical) Chevron Chemical Company or Gulf Oil Company, etc., Ketjen Black EC series (Armak Company), Vulcan XC-72 (Cabot Company) (Cabot Company) and Super P (MMM).

Solvents for forming the electrode include organic solvents such as NMP (N-methyl pyrrolidone), DMF (dimethyl formamide), acetone, dimethyl acetamide or water, and these solvents are used alone or in combination of two or more. It can be mixed and used. However, when forming a cathode, water is used as a solvent. The amount of the solvent used is sufficient to dissolve and disperse the electrode active material, the binder, and the conductive material in consideration of the coating thickness of the slurry and the production yield.

The lithium secondary battery of the present invention may include a separator. The separator is not particularly limited, but it is preferable to use a porous separator, and non-limiting examples include a polypropylene-based, polyethylene-based, or polyolefin-based porous separator.

The lithium secondary battery of the present invention is not limited in appearance, but may be cylindrical, square, pouch type, or coin type using a can.

Hereinafter, the present invention will be described in detail with reference to Examples. However, embodiments according to the present invention can be modified in many different forms, the scope of the invention should not be construed as limited to the embodiments described below. The embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Example  One

Preparation of Nonaqueous Electrolyte

Ethylene carbonate (EC): Ethyl methyl carbonate (EMC) = 3: 7 (v: v) to dissolve LiPF ₆ in an organic solvent to a concentration of 1M, and then to the solution succinic anhydride and tripropyl borate of the formula The nonaqueous electrolyte was prepared by adding 0.5% by weight and 0.1% by weight, respectively, based on the total weight of the nonaqueous electrolyte.

Formula 7

Fabrication of Lithium Secondary Battery

An electrode was prepared using LiCoO ₂ as the positive electrode, artificial graphite as the negative electrode, and SBR as the negative electrode binder. A bicell type pouch battery was manufactured by a conventional method of injecting the nonaqueous electrolyte prepared by the above-described method.

Example  2

A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that tripropyl borane was used instead of tripropyl borate.

Example  3

A nonaqueous electrolyte solution and a lithium secondary battery were manufactured in the same manner as in Example 1, except that 1% by weight of fluoroethylene carbonate represented by Formula 8 was further added.

Formula 8

Example  4

A nonaqueous electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that Tris (trimethyl silyl) borate was used instead of tripropyl borate.

Example  5

A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that Maleic anhydride was used instead of Succinic anhydride.

Example  6

A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that Phthalic anhydride was used instead of Succinic anhydride.

Comparative example  One

A nonaqueous electrolyte solution and a lithium secondary battery were manufactured in the same manner as in Example 1, except that tripropyl borate was not added.

Comparative example  2

A nonaqueous electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 3, except that tripropyl borate was not added.

Comparative example  3

A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that succinic anhydride was not added.

Comparative example  4

A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 2, except that succinic anhydride was not added.

Comparative example  5

A nonaqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 4, except that succinic anhydride was not added.

Comparative example  6

A nonaqueous electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 5, except that tripropyl borate was not added.

Comparative example  7

A nonaqueous electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 6, except that tripropyl borate was not added.

Comparative example  8

A nonaqueous electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 1, except that succinic anhydride and tripropyl borate were not added.

Comparative example  9

A nonaqueous electrolyte solution and a lithium secondary battery were manufactured in the same manner as in Example 1, except that PVdF was used instead of SBR as a negative electrode binder in preparing a lithium secondary battery.

Life characteristic evaluation

Pouch cells according to Examples 1 to 6 and Comparative Examples 1 to 9 were charged and discharged at 0.5 C for 200 times, and the capacity retention ratio was measured in relation to the initial capacity, and the results are shown in Table 1 below.

Table 1

	additive	Capacity retention rate after 200 charge / discharge cycles (%)
Example 1	Succinic anhydride 0.5 wt% tripropyl borate 0.1wt%	85.7
Example 2	Succinic anhydride 0.5 wt% tripropyl borane 0.1 wt%	82.7
Example 3	Succinic anhydride 0.5 wt% Fluoro-ethylene carbonate 1wt% tripropyl borate 0.1 wt%	86.3
Example 4	Succinic anhydride 0.5 wt% Tris (trimethyl silyl) borate 0.1 wt%	81.3
Example 5	Maleic anhydride 0.5 wt% tripropyl borate 0.1 wt%	77.8
Example 6	Phthalic anhydride 0.5 wt% tripropyl borate 0.1 wt%	72.7
Comparative Example 1	Succinic anhydride 0.5 wt%	62.3
Comparative Example 2	Succinic anhydride 0.5 wt% Fluoro-ethylene carbonate 1wt%	65.6
Comparative Example 3	tripropyl borate 0.1 wt%	65.9
Comparative Example 4	tripropyl borane 0.1 wt%	62.2
Comparative Example 5	Tris (trimethyl silyl) borate 0.1 wt%	60.5
Comparative Example 6	Maleic anhydride 0.5 wt%	52.4
Comparative Example 7	Phthalic anhydride 0.5 wt%	44.3
Comparative Example 8	none	51.2
Comparative Example 9	Succinic anhydride 0.5 wt% tripropyl borate 0.1wt% (cathode binder: PVdF)	26.1

As can be seen in Table 1, in the lithium secondary battery in which the nonaqueous electrolyte solution including the cyclic anhydride derivative and the anion receptor at the same time was applied to the negative electrode using the aqueous binder, all of the cyclic anhydride derivative and the anion receptor were not added or each alone It can be seen that the lifespan is greatly improved than when used as.

Meanwhile, although the nonaqueous electrolyte according to the present invention was used, the lithium secondary battery of Comparative Example 9, which used PVdF as a solvent-based binder as a negative electrode binder, was found to have a significantly reduced lifespan. Since the negative electrode using the solvent-based binder has a lower specific surface area than the negative electrode using the aqueous binder, the increase in resistance per unit area due to both additives of the present invention is estimated to have shown this result.

LiF content evaluation of SEI membrane

After charging the battery according to Example 1 and Comparative Example 1 at 0.1C, the negative electrode was detached and subjected to surface analysis by XPS, the results are shown in FIG.

Referring to FIG. 1, the lithium secondary battery of Example 1 in which a nonaqueous electrolyte including a cyclic anhydride derivative and an anion receptor is applied to a negative electrode using an aqueous binder according to the present invention, the nonaqueous electrolyte containing only a cyclic anhydride derivative alone is an aqueous binder. It can be seen that the LiF content in the SEI film is significantly reduced compared to the battery of Comparative Example 1 applied to the negative electrode using. This is because the anion receptor eluted LiF among the components of the SEI film on the surface of the cathode, thereby confirming that an SEI film is easily formed.

Claims (9)

1. In a lithium secondary battery comprising a negative electrode, a positive electrode and a nonaqueous electrolyte,

   The negative electrode includes an aqueous binder,

   The nonaqueous electrolyte,

   (a) cyclic anhydrides or derivatives thereof; And

   (b) A lithium secondary battery comprising any one anion receptor selected from the group consisting of borane compounds, borate compounds and mixtures thereof.
2. The method of claim 1,

   The aqueous binder is a lithium secondary battery, characterized in that SBR (styrene-butadiene rubber).
3. The method of claim 1,

   The cyclic anhydride or a derivative thereof is any one selected from the group consisting of compounds represented by the following Chemical Formulas 1 to 4 or a mixture of two or more thereof.

   <Formula 1>

   

   <Formula 2>

   

   <Formula 3>

   

   <Formula 4>

   

   In Formulas 1 to 4, R ₁ to R ₁₁ are each independently halogen or an alkyl, alkenyl or alkoxy group having 1 to 10 carbon atoms unsubstituted or substituted with halogen.
4. The method of claim 1,

   The borane compound is a lithium secondary battery, characterized in that the compound represented by the formula (5).

   <Formula 5>

   

   In Formula 5, R ₁₂ to R ₁₄ are each independently hydrogen or halogen, or an alkyl group or silyl group having 1 to 6 carbon atoms.
5. The method of claim 1,

   The borate compound is a lithium secondary battery, characterized in that the compound represented by the formula (6).

   <Formula 6>

   

   In Formula 6, R ₁₅ to R ₁₇ are each independently hydrogen or halogen, or an alkyl or silyl group having 1 to 6 carbon atoms.
6. The method of claim 1,

   The content of the component (a) and (b) is a lithium secondary battery, characterized in that 0.05 to 10% by weight, respectively, based on the total weight of the nonaqueous electrolyte.
7. The method of claim 1,

   The non-aqueous electrolyte lithium secondary battery, characterized in that it comprises a lithium salt and a carbonate organic solvent.
8. The method of claim 7, wherein

   The lithium salt is LiPF _{6 ,} LiBF _{4 ,} LiSbF _{6 ,} LiAsF _{6 ,} LiClO ₄ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , CF ₃ SO ₃ Li, LiC (CF ₃ SO ₂ ) Lithium secondary battery, characterized in that any one selected from the group consisting of ₃ , and _, LiC ₄ BO ₈ or a mixture of two or more thereof.
9. The method of claim 7, wherein

   The carbonate organic solvent is linear selected from the group consisting of cyclic carbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, dipropyl carbonate and methylpropyl carbonate selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate and fluoroethylene carbonate Lithium secondary battery, characterized in that the carbonate or a mixture thereof.

Images